Condition-responsive control circuit



June 1, 1965 H. 'r. ADKINS 3,187,204

CONDITI ON-RESPONSIVE CONTROL CIRCUIT- Filed June 13, 1963 l7 l4- o '3 RELAY A RELAY (6 f CONTACTS VARISTOR IN V EN TOR.

Harold 7T Adkins BY MOSE8,Mc6/Ew a; 75am United States Patent 3,187,204 CONDITION-RESPONSIVE CONTROL CIRCUIT Harold T. Adkins, Marshfield, Mass, assignor to Sigma Instruments, Inc., a corporation of Massachusetts Filed June 13, 1963, Ser. No. 287,701 6 Claims. (Cl. 307-417) This invention pertains, in general, to condition-responsive load-controlling circuitry; and, in particular, to a control circuit for connecting or disconnecting a load and a power source in response to a predetermined level of physical condition, the control circuit being voltageregulated, transient-protected and easily adapted for use with power sources having different supply voltages.

Although the subject control circuit is described hereinafter as being particularly applicable for turning on and turning oil lamps in response to predetermined levels of light, it is to be understood that the condition initiating the control action may be other than a level of illumination and that the load to be controlled may be other than a lighting load. For example, the condition initiating the control action may be a temperature level, pressure level, humidity, etc.

One object of the present invention is to provide a control circuit which is voltage-regulated.

A further object of the invention is to provide a control circuit which is protected against surges or transients.

Another object of the invention is to provide a control circuit which may easily be adapted for use with differentpower sources having different supply voltages.

Another object of the invention is to provide a condition-initiated load-controlling circuit which is simply conructed, reliable and economical.

According to one illustrative embodiment of the invention, there is provided a first circuit comprising a current-limiting resistor, a contact-actuating relay coil, and a photoconductor, all connected in series across a voltage source. Shunted across the combination of the relay coil and the photoconductor is a varistor, which is also connected in series With the current-limiting resistor. Also connected across the voltage source is a second circuit comprising a load to be controlled and a set or" contacts in series therewith, the contacts being operable by energization of the relay coil.

One feature of the invention is the employment of a symmetrical varistor connected across the series combination including both the relay coil and the photoconductor. As is known, the symmetrical varistor is a semiconductor device which is characterized by a relatively small voltage drop variation attending extremely large current variations therethrough. Expressed mathematically, this characteristic can be expressed as follows:

In the above equation, I represents current passing through the varistor in amperes; V represents the voltage drop across the varistor in volts; n is a dimensionless exponent which is greater than 1; and K is a constant of proportionality.

The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its use, ref erence should be had to the accompanying drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.

In the drawing figure there is illustrated, schematically, a control circuit according to the invention. As shown, the terminals 10 and 12 are adapted to be connected across an alternating voltage supply source. A load to be controlled, such as a and 12 include,

3,187,204 Patented June 1, 1965 group of luminaires, is intended to be connected between the terminals 11 and 12. Serially connected between the terminal 11 and junction 13 is the set of normally-closed contacts 16a. The contacts 16a are arranged to be opened when sufiicient current passes through the relay coil 16. Although the set of contacts 16a is illustrated in the drawing as being in a normally closed condition, it is to be understood that, if desired, a set of normally-open contacts could be employed, depending upon whether the load, connected between the terminals 11 and 12, is intended to be deenergized, or energized, when relay coil 16 has sutficient current passing therethrough.

The main conductors 14 and 15 which are connected across the voltage source by means of the terminals 10 as shown, a first series circuit comprising the current limiting resistor 17, the relay coil 15, and

, phide cell. prised of silicon carbide or the like.

the photoconductor 18. Shunted across the relay coil 16 and the photoconductor 18 is the symmetrical varistor 19.

Preferably, the photoconductor 18 is a cadmium sul- The symmetrical varistor 19 may be com- Symmetrical varistors of silicon carbide are well known semiconductor devices. See, for example, the text Semiconductor Devices, authored by John N. Shive and published by D. Van Nostrand Company, Inc., copyright 1959, pp. 9194.

Operationally the control circuit, hereinbefore described and illustrated in the drawing figure, functions as follows:

Under normal conditions (i.e., a normal voltage input between the terminals 10 and 12 and in the absence of transient or surge voltages) the relay coil 16 will not have suflicient current passing therethrough to actuate the set of contacts 16a when the ambient illumination incident on the photoconductor cell 18 is at a predetermined low or dark level. However, at a predetermined higher level of illumination (sufficient daylight), the resistance of the photoconductor cell 18 drops to a relatively low value, thereby permitting sufficient current to flow through the resistor 17, the relay coil 16 and the cell 18, thereby sufficiently energizing the relay coil 16. As a consequence, the set of contacts 16a opens. Accordingly, the load circuit connected between the terminals 11 and 12 is disconnected rrom the voltage source.

When variable voltages, including surge or transient voltages, are applied to the control circuit, the varistor 19 and the current limiting resistor 17 will function as a non-linear voltage divider to maintain the voltage across the relay coil 16 and photoconductor cell 18 within an appropriate range. A symmetrical varistor having the following volt-ampere characteristics may be used:

At a voltage drop of 62 volts across the symmetrical varistor, the current passing therethrough is 0.001 ampere. The same varistor having a voltage drop of 124 volts thereacross has a current of 0.01 ampere passing therethrough. Also, the same varistor having a voltage drop of 250 volts thereacross has a current of 0.15 ampere passing therethrough.

In general, it is preferred to employ a symmetrical varistor characterized by wherein n=4. However, a varistor wherein n is in the range of from 3 to 5 is suitable.

The series combination of current-limiting resistor 17 and varistor 19 has a heat-dissipation capacity which is several tmes that of the combination of relay coil 16 and photoconductor 18. In addition, the use of the varistor 19 enables dispensing with the provision of the usual lightning arrestor, due to the fact that the arrangement of current-limiting resistor 17 and varistor 19 can adequately handle very large transient voltages.

eas /e04 While a specific embodiment of the invention has been shown and described in detail to illustrate the application of the inventive principles, it will be understood that the invention may be embodied otherwise without departing from such principles.

What is claimed is: i

i. A circuit for controlling the flow of electric power between a load and a power source in response to a predetermined condition level comprising: a pair of terminals constructed and arranged for connection to power sources at different voltages, contact means connected, in'series with the load, across said terminals; circuit means connected across said terminals and including impedance means connected in series with a circuit combination including, current operable means for operating said contact means and condition sensing means responsive to a predetermined condition level for enabling an effective current flow to operate said current operable means; and voltage regulating means connected in shunt with the combination of said current operable means and the condition sensing means, and in series with said impedance means for maintaining the voltage across the combination of said current operable means and condition sensing means within predetermined relatively narrow limits when the voltage across said terminals fluctuates.

. 2. A circuit, for controlling the flow of electric power between a load and a power source in response to a predetermined illumination level, comprising: a pair of terminals constructed and arranged for connection to power sources of different vo1tages,contact means connected, in

series with the load, across said terminals; circuit means connected across said terminals, said circuit means including. impedance means connected in series with a circuit combination including, a relay coil operable in response to current therethrough for operating said contact means and a photoconductor responsive to a predetermined illumination level for enabling the passage of an effective current through the relay coil; and a symmetrical varistor connected in shunt with the combination of said relay coil and photoconductor, and in series with said impedance means.

3. A circuit, for controlling the flow of electric power between a load and a power source in response to a predetermined illuminationlevel, comprising: a pair of terminals constructed and arranged for connection to power sources having different voltages; contact means connected, in series with the load, across said terminals; a non-linear voltage divider including a current-limiting resistor connected to one of said terminals and a varistor connected in series between said resistor and the other of said terminals; a relay coil for operating said contacts; a photoconductor connected in series with said relay coil; the series combination of said relay coil and said photoconductor being connected between said other terminal and the junction of said resistor and said varistor, whereby said varistor is connected in shunt with the series combination of said relay coil and said photoconductor; said photoconductor, responsive to a predetermined illumination level, enabling passage of an effective contact operating current through said relay coil; said varistor maintaining the voltage across the series combination of said relay coil and said photoconductor within predetermined relatively narrow limits irrespective of variations in the voltage across said terminals.

4. A circuit, according to claim 3, wherein said varistor is a symmetrical varistor defined by l=KV where 1 represents current, in amperes, passing through the varistor; V represents the voltage drop in volts, across the varistor; K represents a constant of proportionality; and n represents a dimensionless exponent, it having a value in the. range of from 3 to 5.

5. A circuit, according to claim 3, wherein said varistor is a symmetrical varistor defined by 1=KV; where T represents current, in amperes, passing through the varistor; V represents the voltage drop, in volts, across the varistor; K represents a constant of proportionality; and It has a value a value of approximately 4. v

, 6. A circuit, for controlling the flow of electric power between a load and power source in response to a predetermined illumination level, comprising: a pair of terminals constructed and arranged for connection to power sources having different voltages; contact means connected, in series with the load, across said terminals; a relay coil, for operating said contacts, and a photoconductor connected across said terminals; said photoconductor, responsive to a predetermined illumination level,

enabling the passage of an etiective contact operating current through said relay coil; a varistor connected across said terminals in shunt with the combination of said relay coil and said photoconductor; and current-limiting means in series with said varistor and in series with the combination of said relay coil and said photoconductor; said varistor and said current-limiting means maintaining the voltage across the combnation of said relay coil and said photoconductor within predetermined relatively narrow limitsirrespective of variations in the voltage across said terminals.

Reierences Cited by the Examiner UNITED STATES PATENTS 2;900,520 8/59 Frank 250-206 2,927,213 3/60 Marion et al'. 250-209 2,967,981 1/61 Wise 250-206 X 3,048,833 8/62 Bernheim 250-206 X 3,080,491 3/63 Howell 250-206 X 3,081,417 3/63 Collier 250-206 X LLOYD MCCOLLUM, Primary Examiner. RALPH o. NILSON, Examiner. 

3. A CIRCUIT, FOR CONTROLLING THE FLOW OF ELECTRIC POWER BETWEEN A LOAD AND A POWER SOURCE IN RESPONSE TO A PREDETERMINED ILLUMINATION LEVEL, COMPRISING: A PAIR OF TERMINALS CONSTRUCTED AND ARRANGED FOR CONNECTION TO POWER SOURCES HAVING DIFFERENT VOLTAGES; CONTACT MEANS CONNECTED, IN SERIES WITH THE LOAD, ACROSS SAID TERMINALS; A NON-LINEAR VOLTAGE DIVIDER INCLUDING A CURRENT-LIMITING RESISTOR CONNECTED TO ONE OF SAID TERMINALS AND A VARISTOR CONNECTED IN SERIES BETWEEN SAID RESISTOR AND THE OTHER OF SAID TERMINALS; A RELAY COIL FOR OPERATING SAID CONTACTS; A PHOTOCONDUCTOR CONNECTED IN SERIES WITH SAID RELAY COIL; THE SERIES COMBINATION OF SAID RELAY COIL AND SAID PHOTOCONDUCTOR BEING CONNECTED BETWEEN SAID OTHER TERMINAL AND THE JUNCTION OF SAID RESISTOR AND SAID VARISTOR, WHEREBY SAID VARISTOR IS CONNECTED IN SHUNT WITH THE SERIES COMBINATION OF SAID RELAY COIL AND SAID PHOTOCONDUCTOR; SAID PHOTOCONDUCTOR, RESPONSIVE TO A PREDETERMINED ILLUMINATION LEVEL, ENABLING PASSAGE OF AN EFFECTIVE CONTACT OPERATING CURRENT THROUGH SAID RELAY COIL; SAID VARISTOR MAINTAINING THE VOLTAGE ACROSS THE SERIES COMBINATION OF SAID RELAY COIL AND SAID PHOTOCONDUCTOR WITHIN PREDETERMINED RELATIVELY NARROW LIMITS IRRESPECTIVE OF VARIATIONS IN THE VOLTAGE ACROSS SAID TERMINALS. 